The BIG difference:
Rooftop A/C: $1000-$1500
Portable A/C: $179-$299
When you don't have a good place for a cheap window unit, and absolutely cannot justify the $$$ for a rooftop unit, the portable is a good alternative.
They *DO* work. Sure, they won't get it cold enough to store raw meat per USDA regulations, but they do cool things down and pull the humidity out of the air.
They do take up floor space. For the price difference, though, I for one decided that I would deal with that mild inconvenience. The unit makes a GREAT table.
Better than laying awake all night, stewing in your own juices.